1. Field of the Invention
The present invention generally relates to a nitride-based semiconductor device with a diode structure and a nitride-based semiconductor device with a diode and a field effect transistor provided on a same substrate.
2. Description of the Related Art
A power semiconductor device such as a switching device and a diode is used for a power conversion device such as a switching power supply. A high breakdown voltage, a low on-voltage, and a low reverse-bias leak current are demanded for the power semiconductor device. To obtain the high breakdown voltage, it is effective to use a material having high critical electric field. From these facts, a nitride-based semiconductor device made of a nitride-based semiconductor material is useful as a semiconductor device which is used in the power conversion device.
As a conventional nitride-based semiconductor device, a field effect transistor structured in the following manner has been known. The field effect transistor is such that a carrier traveling layer made of AlXGa1-XN (0≦X<1) film and a barrier layer made of AlYGa1-YN (0<Y≦1, X<Y) film are successively laminated, and that a gate electrode is formed at a predetermined position on the surface of the barrier layer having a uniform thickness, and a source electrode and a drain electrode are formed at positions nearly symmetric across the gate electrode.
The AlN film has a lattice constant smaller than that of the GaN film. Therefore, when the Al composition ratio in the barrier layer is greater than the Al composition ratio in the carrier traveling layer, the lattice constant of the barrier layer becomes smaller as compared with that of the carrier traveling layer, and a distortion occurs in the barrier layer. In the nitride-based semiconductor, piezoelectric charge is produced in the barrier layer due to the piezo effect caused by the distortion in the barrier layer. The piezoelectric charge produced at that time causes a two-dimensional electron gas to be formed at the interface between the carrier traveling layer and the barrier layer.
For example, when the GaN film having the Al composition of X=0 is used as the carrier traveling layer and the AlYGa1-YN film is used as the barrier layer, a carrier density ns of the two-dimensional electron system to a film thickness d1 of the barrier layer is obtained from the following equation (1) (J. P. Ibbetson et al., “Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors”, Applied Physics Letters, 10 Jul. 2000, Vol. 77, No. 2, P. 250-252).ns=σPZ×(1−Tc/d1)[cm−2]  (1)
Where σPZ is a charge density of piezoelectric charge produced in the barrier layer, and d1 is a thickness of the barrier layer below the gate electrode. Further, Tc is a critical thickness of the barrier layer in which the carrier is generated. The critical thickness Tc is given by the following equation (2), and shows a dependency to the Al composition.Tc=16.4×(1−1.27×Y)/Y[Å]  (2)
As a device made of the nitride-based semiconductor material, a diode as follows can be considered. The diode is such that the carrier traveling layer made of a GaN film in which Al composition is X=0 and the barrier layer made of AlYGa1-YN (0≦Y≦1) film are successively laminated, and that an anode electrode having a Schottky contact and a cathode electrode having an ohmic contact which is arranged so as to surround the anode electrode are formed on the barrier layer with a predetermined thickness.
When the anode electrode with the Schottky contact is produced using metal such as Ni or Pt on the barrier layer, a Schottky barrier height is about 1 electron volt (eV). And then, a forward bias is applied to the anode electrode, and the nitride-based semiconductor device structured as above is caused to perform diode operation. An on-state voltage upon the diode operation becomes high such as about 1 volt (V), which is almost as high as the Schottky barrier height.
To lower the on-state voltage, the Schottky barrier height needs to be decreased. To decrease the Schottky barrier height, a method of replacing the material of the anode electrode with a metal having a low work function can be considered, but in this case, a reverse-bias leak current is increased. According to a calculation allowing for specific thermal emission, if the on-voltage is lowered by 0.1 V, the reverse-bias leak current increases by one digit or more. In other words, it is difficult for the nitride-based semiconductor device, which includes the anode electrode having the Schottky contact and the cathode electrode having the ohmic contact, to lower the on-voltage while keeping the reverse-bias leak current to be low.